Conventionally, in an automatic tool changer of a numerical control machine tool, there has been proposed a device to monitor an anomaly in tool change operation of a tool change arm (see, e.g., Japanese Patent Laid-open Application No. H11-333657). The device detects torque waveform pattern of a driving motor generated when the tool change arm is at various positions on a track thereof being made while performing the change operation. The detected torque waveform pattern is compared with that of the driving motor generated while the tool change arm operated normally, thereby detecting an anomaly in the tool change operation. As for the comparison of the torque waveform patterns, an amount of error in a torque value of the tool change arm performing the change operation is computed at every identical positions and it is checked whether the amount of error is within an allowable range or not to judge an anomaly in the tool change operation.
The device in the Patent Reference supra accomplishes the technical task of preventing damages of a machine tool and a workpiece by monitoring whether an anomaly caused by a fabrication error in the tool change arm exists in the tool change operation or not. In the Patent Reference, torque waveform patterns for a plurality of tools held in a tool magazine are pre-obtained in consideration of weight differences thereof and are stored. Then, when a tool is replaced with another, a torque waveform pattern corresponding to the another tool is used.
The technology in Patent Reference is for detecting an anomaly in the automatic tool changer. Therefore, a tool is regarded as being attached properly to the driving unit if the automatic tool changer is normally operating. However, even if the tool is adequately attached to the driving unit not to incur difficulties in machining a workpiece, some errors in a machining dimension can occur due to, e.g., misalignment of shaft axis, and improper attachment wear, and broken edges of the tool. An acceptable range of error is defined in terms of a tolerance. Therefore, there is no problem if the machining dimension is within the tolerance range, but if the machining dimension is out of the tolerance range, the machined workpiece is deemed to be defective. The machining dimension can be known only after completing the machining, resulting in a waste of workpiece.